I doubt it the gas cap because when the code says sensor in it it is usually and electrical problem most of the time but not always. If it was a leak from the cap it should have a code like "Evap Leak" since it would not hold pressue or vacum depending on what year it is. When the code says "Evap Sensor" it normally is not seeing a change in the sensor reading or not seeing and current flow thru
the sensor circuit. The controller will do a test first to make sure the sensor works as it will pump up the LDP or pull a vacum looking for the switch to show a change and verify it works. Once that passes it will do a leak test which is normally if it fails set a code like Evap system leak. If you tell us what year it is we may be able to look the code up for that year and give you the normal reasons for that exact code. It could still be a leak as there are times the PCM even gets fooled. Ron